The present invention relates to the transmission of radio frequency signals which have high frequency and wide bandwidth requirements and transmission of optical information over fiber optic data links with a transmitter that utilizes a logarithmic video detector/limiter amplifier.
Optical data signals have been sent over fiber optics by directly modulating a laser diode with a signal from an electrical signal source. Directly modulating the laser diode with a signal source results in a limited bandwidth due to parasitic conductance and capacitance effects. These effects are caused by frequency changes that occur due to rapid variation in the current in the laser diode.
Distributed feedback lasers have been recently developed which reduce such frequency changes, but the dynamic range limitation of conventional lasers that are amplitude modulated is typically limited to 30 to 50 dB due to the harmonic distortion that is produced in the laser which is an inherent limitation of direct modulation.
Optical modulation and transmission of r.f. signals with a large high frequency range and a large bandwidth are provided by the present invention in which the electrical information-bearing signal is supplied to a logarithmic video detector/radio frequency (r.f.) limiting amplifier, which supplies an amplitude-limited carrier output and a second output that is a result of the logarithmic gain response of the amplifier.
Logarithmic video detector/r.f. limiting amplifiers can provide two output signals, one of which is a constant amplitude carrier signal version of the input signal and the other of which has an amplitude that is logarithmically proportional to the amplitude envelope of the input signal. These two signals may each be processed in different manners such that one signal may be converted to an optically modulated signal and the other may remain an electrical signal. Transmission of both types of signals over a common cable may be desirable. Such cables are known.
U.S. Pat. No. 4,158,478 discloses a combined optical/electrical cable which requires a central conductor, a surrounding shield and a plurality of fibers embedded in the dielectric around the central conductor.
U.S. Pat. No. 4,695,127 shows a combined optical/electrical conductor in which a single electrical conductor and a single optical fiber is loosely wound about the central conductor (see column 3 lines 16-34) which would be more cost effective for the above-noted application. However, this cable requires some form of an additional spacer which complicates the structure of the cable and requires additional components even in the simplest configuration, as shown in the patent, in which a space is spirally wound about the central conductor and an additional buffer tube is required inside the shield.